(1) Field of the Invention
In modern aircraft, cruising elevations of 35,000 ft. or greater are not uncommon. While the atmospheric pressure outside the aircraft becomes very low at such altitude, it is required that the pressure inside the aircraft cabin remain as near sea level pressure as possible to provide adequate oxygen for the passengers. However, if cabin pressure is not permitted to decrease, the pressure difference between inside cabin pressure and outside ambient pressure can become sufficiently great at high altitude to cause a catastrophic rupturing of the aircraft. Accordingly, it has been standard practice to permit cabin pressure to decrease to a value corresponding to an altitude of about 8,000 ft. Thus, structural integrity of the aircraft can be maintained while providing adequate oxygen for passenger breathing.
This variation in cabin pressure must be accomplished without sacrificing passenger comfort. Since the human ear is more sensitive to increases in pressure (descent in elevation) than to decreases in pressure (ascent in elevation), the passenger comfort factor is complicated by the need for different permissible maximum change rates, for use in each phase of operation. Furthermore, for maximum passenger comfort the cabin pressure should not be subject to pressure spikes or changes when the aircraft momentarily climbs or drops in altitude.
(2) Prior art
The importance of cabin pressure control when viewed in light of passenger comfort and safety has imposed a great burden on the flight crew work load. This burden is ever increasing while the present tendency is to reduce the size of the flight crew. For these reasons, methods of automatic cabin pressure control have been developed which require minimum work by the flight crew. However, these prior art automatic cabin pressure control systems are known to have significant deficiencies. Although flight crew work load is greatly reduced, the attention of one member of the flight crew periodically must be focused on the cabin altimeter to clock the rate of cabin pressure change or to compare it with the aircraft altimeter to be assured that the single automatic cabin pressure controller is functioning properly. Also, automatic controllers in general will compute a rate of cabin pressure change which is a function of the differential between existing cabin pressure and its final value or the existing aircraft altitude and its final value. Previously chosen references upon which the rate of cabin pressure was based resulted in a rate of change which was subject to instantaeous rapid changes when the aircraft altitude would change rapidly due to air pockets or foul weather. Examples of this prior type of automatic cabin pressure controller are found in U.S. Pat. No. 3,473,460 to F. R. Emmons and U.S. Pat. No. 3,461,790 to R. C. Kinsell.